Examples of a water-soluble ink for use in a printer of an ink jet recording type that ejects ink droplets through fine nozzles in response to an image signal while controlling the flying route of the ink droplets which are then attached to the surface of paper such as ordinary paper to provide recording include those disclosed-in JP-A-2-255774 (The term "JP-A" as used herein means an "unexamined published Japanese patent application") and JP-A-6-25573 (corresponding to U.S. Pat. No. 5,395,434).
The water-soluble ink for ink jet type recording as disclosed in JP-A-2-255774 comprises water, a water-soluble dye, a drying inhibitor and a penetrant, wherein the penetrant comprises a compound represented by the following formula: ##STR3## wherein R.sup.1 represents hydrogen atom, methyl group, methoxy group, chlorine atom or bromine atom; and n.sup.1 represents an integer 1 to 5 (hereinafter referred to as compound (1))
and a butyl alcohol-ethylene oxide adduct having an ethylene oxide addition mole number of 1 to 5 (hereinafter referred to as "compound (2)"), and optionally a butyl alcohol-propylene oxide adduct having an ethylene oxide addition mole number of 1 to 5 (hereinafter referred to as "compound (3)"), whereby the surface tension of the water-soluble ink is controlled within a range of from 30 to 40 dyne.
Ink obtained by mixing the foregoing various compounds may be continuously and stably ejected against a commercial copying paper in a drop-on-demand type jet printer for a prolonged period of time. She ink also can thoroughly penetrate into an ordinary paper, whereby a good print image quality can be provided.
The ink for ink jet type recording as disclosed in JP-A-6-25573 comprises a dye, water, 0.1 to 20% by weight of a compound represented by the following formula: ##STR4## wherein R.sup.2 represents an alkyl group, and n.sup.2 represents an integer (hereinafter referred to as "compound (4)"), (hereinafter referred to as "compound (5)"), and urea or its derivative (hereinafter referred to as "compound (6)"), whereby a uniform image free of color deviation can be provided without causing bleeding (i.e., mixing of different colors caused by unfixed ink dots when they are brought into contact with each other) even when a high speed printing is made on an ordinary paper.
The foregoing conventional aqueous ink for ink jet type recording comprises compound (1), and compound (2), and optionally compound (3) as a penetrant. The aqueous ink causes no clogging in the printer nozzle and thus can be stably ejected to provide a high printing quality. Further, the aqueous ink comprising compounds (4), (5) and (6) are said to exhibit an improved wettability with respect to paper and thus causes less bleeding. However, these conventional aqueous inks are disadvantageous in that they cause feathering (i.e., oozing out of ink along paper fibers) and thus cannot provide a printed image with clear out-line when used with poor quality paper having a low fiber density than ordinary paper (e.g., regenerated papers, paper for use in a register printer).
Further, since the fibers (chief material mainly composed of cellulose) and a sizing agent (agent for smoothening the paper surface, mainly composed of starch or rosin) constituting the paper are oily materials, the foregoing aqueous inks, which have a low lipophilicity, are disadvantageous in that ink I attached to paper P hardly penetrate into paper P and thus gives a small diameter of printed dot d1 as shown in FIG. 1A, making it impossible to provide a sharp printed image. When the lipophilicity of the ink is increased to solve these difficulties, the penetrant and other components are rendered insoluble in water or, if soluble, the resulting ink becomes foamable, whereby it is impossible to provide stable ejection (continuous ejection of ink droplets in a constant amount at a constant rate).
On the other hand, a relatively small-sized wire dot printer, for example, is equipped with a printing head weighing about 60 g. Therefore, it is possible to apply inexpensive and small-sized printer driving devices into an ink jet type printer as it is if the weight of the ink unit (excluding the weight of the recording head and the cartridge) is not more than about 50 g, as one possibility. Further, in view of the life of the ink ribbon in the existing wire dot printer, it is desirable that 10 million letters can be printed with an ink unit weighing 50 g or less to make the ink jet type printer applicable to various applications of use. However, since the conventional aqueous ink must be consumed in a large amount to make printed letters visible, the ink unit tends to enlarge to a weight of 50 g or more, which renders an ink jet type printer to be large. On the other hand, if the amount of the ink is reduced to a weight of 50 g or less, it causes frequent replacement of the ink jet cartridge, whereby its facility in use deteriorates.